1. Field of the Invention
The present invention relates to a semiconductor device used for power control. More specifically, the invention relates to a horizontal power FET (field-effect transistor) and an SBD (Schottky barrier diode).
2. Description of the Related Art
Conventionally a power semiconductor device such as a switching device and a diode has been used in a switching power supply, an inverter circuit and the like. The power semiconductor device needs to be high in breakdown voltage and low in on-resistance. However, there is a tradeoff, which depends upon device materials, between the breakdown voltage and the on-resistance in the power semiconductor device.
In accordance with the advance of technical development, the on-resistance of a power semiconductor device is lowered to the vicinity of the limit of electrical resistance of silicon that is the principal material of the power semiconductor device. It is thus necessary to change the material in order to lower the on-resistance further. A power semiconductor device has recently been proposed which employs a nitride semiconductor such as gallium nitride (GaN) and aluminum gallium nitride (AlGaN) and a wideband gap semiconductor such as silicon carbide (SiC) as switching device materials. A power semiconductor device using such a wideband gap semiconductor can improve in the above tradeoff that depends upon device materials and dramatically decrease in on-resistance (see, for example, N. -Q. Zhang et al., “High Breakdown GaN HEMT with Overlapping Gate Structure,” IEEE Electron Device Letters, Vol. 21, No. 9, September, 2000).
However, when a horizontal power device is formed of a wideband gap semiconductor, if the breakdown voltage of a surface passivation insulating film is low, the breakdown voltage of the device will depend upon the breakdown voltage and the device will be broken the instant that a voltage higher than the breakdown voltage of the surface passivation insulating film is applied to the device. In order to avoid this, the device is designed to decrease the electric field therein and have an adequate breakdown voltage. Such a design however makes it impossible to bring about the capability of the wideband gap semiconductor and results in increase in on-resistance.
The above device has another problem of having no avalanche capability because the device is broken before an avalanche breakdown occurs.